highly flexible conductive fabrics with hierarchically ... · highly flexible conductive fabrics...

Nano Res

1

Highly flexible conductive fabrics with hierarchically

nanostructured amorphous nickel tungsten tetraoxide

for enhanced electrochemical energy storage

Goli Nagaraju1, Ramesh Kakarla2, Sung Min Cha1, and Jae Su Yu1 ()

Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0874-z

http://www.thenanoresearch.com on August 4, 2015

© Tsinghua University Press 2015

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Nano Research

DOI 10.1007/s12274-015-0874-z

TABLE OF CONTENTS (TOC)




Goli Nagaraju1, Ramesh Kakarla2, Sung Min Cha1 and Jae

Su Yu1*

1Department of Electronics and Radio Engineering,

Institute for Wearable Convergence Electronics, Kyung

Hee University, 1 Seocheon-dong, Giheung-gu, Yongin-si,

Gyeonggi-do 446-701, Republic of Korea,

2Department of Environmental Science and Engineering,

Kyung Hee University, 1 Seocheon-dong, Giheung-gu,

Yongin-si, Gyeonggi-do 446-701, Republic of Korea

Burl-like

amorphous

NiWO4 NSs

Amorphous NiWO4

nanostructures

(i.e., NSs) on CF

Cu layer PET fiber

Hierarchical NSs

Amorphous NiWO4 NSs with burl-like morphologies were facilely

integrated on flexible CF fibers using ED method.

As a flexible and cost-effective electrode for psuedocapacitors, the

hierarchically nanostructured amorphous NiWO4/CF exhibited superior

electrochemical properties.




Goli Nagaraju1, Ramesh Kakarla2, Sung Min Cha1 and Jae Su Yu1 ()

Received: day month year

Revised: day month year

Accepted: day month year

(automatically inserted by

the publisher)

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2014

KEYWORDS

Amorphous NiWO4

nanostructures,

Conductive fabrics,

Electrochemical

deposition,

Electrochemical energy

storage properties

ABSTRACT

Amorphous nickel tungsten tetraoxide (NiWO4) nanostructures (NSs) are

successfully synthesized on flexible conductive fabric (CF) using a facile

one-step electrochemical deposition (ED) method. With an applied external

cathodic voltage (-1.8 V for 15 min), the amorphous NiWO4 NSs with burl-like

morphologies are well adhered on the seed coated CF substrate. The burl-like

amorphous NiWO4 NSs on CF (i.e., NiWO4 NSs/CF) are employed as a flexible

and binder-free electrode for psuedocapacitors, which exhibit remarkable

electrochemical properties with high specific capacitance (1190.2 F/g at 2 A/g),

excellent cyclic stability (92 % at 10 A/g), and good rate capability (765.7 F/g at

20 A/g) in 1 M KOH electrolyte solution. The superior electrochemical

properties can be ascribed to the advantageous properties of hierarchical and

large specific surface area of burl-like amorphous NiWO4 NSs/CF. This

cost-effective facile method for the synthesis of metal tungsten tetraoxide

nanomaterials on flexible CF could be a promising electrode for advanced

electronic and energy storage device applications.

1. Introduction

The growing interest in the development of

high-performance textile-based energy storage and

conversion devices has attracted widespread

attention due to their advanced feasibilities including

lightweight, high flexibility, low cost, and wearable

ability [1-5]. Among various flexible energy storage

devices, supercapacitors with irreplaceable

properties, such as high power density, fast

charge-discharge capacity, long lifetime, eco-friendly

operation, and low cost, have considered as the most

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2 Nano Res.

promising next-generation energy storage device to

satisfy the rapidly increasing power demand [6-10].

Accordingly, these supercapacitors have started to

play an indispensable role in the growth of

automotives and portable electronic systems

including hybrid electric vehicles, memory back-up

equipments, microelectromechanical systems, digital

cameras, and cellular phones [11-14]. With regard to

the energy storage mechanism, supercacpitors can be

classified into two types, i.e., electric double-layer

capacitors (EDLCs) and psuedocapacitors or redox

supercapacitors [15]. Typically, in EDLCs, the energy

can be stored by the electrostatic diffusion and

accumulation of ionic charges at the

electrode/electrolyte double layer interface [16]. In

contrast, psuedocapacitors can store more energy

than EDLCs due to the fast faradic charge transfer

reactions occurring between electroactive material

and electrolyte solution [17]. Therefore, intense

research studies have been devoted to the fabrication

of nanostructured single and mixed metal

oxides/hydroxide electroactive materials with

bespoke morphologies to increase the energy storage

abilities for psuedocapacitors [18-22]. During the

electrode preparation for psuedocapacitors, mixing

of polymer binders and conductive agents with

electroactive materials causes an increase of

dead-surface. This increased dead surface can

suppress the electron transfer as well as electrolyte

diffusion, which may limit their charge storage

properties. Therefore, the direct growth of

electroactive materials onto the conductive substrates

produces binder- and conductive additive-free

electrode for high-performance psuedocapacitors by

getting rid of the above mentioned limitations and

increases the electroactive sites for reversible faradic

redox reactions between nanomaterials and

electrolyte solution [23-26]. Various growth methods

including hydrothermal synthesis, solvothermal

method, physical deposition, and electrochemical

deposition (ED) have been employed to fabricate

electroactive materials directly on specific conductive

substrates based on conventional psuedocapacitor

electrodes [27-30]. Of these methods, the ED process

is versatile and simple to deposit various

nanostructured materials on the conductive

substrates by the help of an applied external cathodic

voltage in the growth solution. Moreover, the shape

and size of the nanomaterials can be readily tuned by

controlling the applied external cathodic voltage at

low growth temperature for short time [31].

Meanwhile, novel conductive substrates such as

nickel (Ni) foam, titanium (Ti) foil, carbon cloth, and

Ni foil have received much attention as a potential

candidate for psuedocapacitor electrodes [32-34]. As

a matter of fact, the cost of these conductive

substrates is highly expensive due to their

complicated manufacturing processes. Thus, the

conductive fabric (CF) substrates, fabricated by

simple electroless plating of metallic layer on

polyethylenterephthalate (PET) fibers, have recently

used as a low-cost conductive electrode to replace the

expensive current collectors in psuedocapacitors.

Owing to the process easiness, the CF substrate has

beneficial properties (e.g., good flexibility,

conductivity, and mechanical stability) similar to the

conventional psuedocapacitor electrodes [35, 36].

On the other hand, traditional single transition metal

oxides/hydroxides materials including MnO2 [37],

RuO2 [38], Co3O4 [39], NiO [40], Fe2O3 [41], TiO2 [42],

Co(OH)2 [43], and Ni(OH)2 [21] and nanostructures

based on transition binary metal oxides/hydroxides

like NiCo2O4 [12], NiMoO4 [44], MnCo2O4 [45], Ni-Co

[46], and Ni-Al layered double hydroxides [47] have

been extensively explored as pseudocapacitive

materials due to their multiple oxidation states for

reversible electrochemical reactions. Cobalt tungsten

tetraoxide (i.e., CoWO4) and nickel tungsten

tetraoxide (NiWO4) are important inorganic

functional materials for diverse potential applications

[48-50]. Additionally, these materials offer excellent

electrochemical properties as pseudocapacitive

materials in energy storage devices, which can be

ascribed to their good reversible faradic redox

reactions and good conductivity on the order of 10-7

to 10-3 S cm-1 by the incorporation of W atoms. And,

the amorphous nature of these metal tungsten

tetraoxide nanomaterials ensures unique energy

storage properties than their crystalline counterparts

[51, 52]. Thus, to further improve their

electrochemical energy properties, the direct growth

of these metal tungsten tetraoxide nanomaterials on

low-cost conductive substrates by facile growth

methods are highly required. However, there is no

such report up to now, which is based on simple ED

process of metal tungsten tetraoxide

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

3 Nano Res.

pseudocapacitive materials on flexible textile

electrodes and their electrochemical properties. Here,

our objective is to grow the amorphous metal

tungsten tetraoxide nanomaterials on low-cost

flexible electrodes using a simple two-electrode

system based ED method, together with analysis of

their charge storage properties, for psuedocapacitors

without using any polymer binders and conductive

agents.

In this work, we successfully fabricated the burl-like

amorphous NiWO4 nanostructures (NSs) on CF

substrate (i.e., NiWO4 NSs/CF) via a facile ED

method and its application as a flexible and

cost-effective electrode for psuedocapacitors. The

burl-like amorphous NiWO4 NSs were densely and

abundantly coated on CF substrate with good

adhesion by applying an external cathodic voltage at

low growth temperature for short time. The

electrochemical properties reveal that the

as-prepared amorphous NiWO4 NSs/CF show high

specific capacitance and excellent capacity rate as

well as good cycling performance.

2. Experimental section

2.1. Materials

Nickel nitrate hexahydrate (Ni(NO3)2·6H2O), sodium

tungstate dehydrate (Na2WO4·2H2O), nickel acetate

tetrahydrate (Ni(CH3CO2)2·4H2O), and sodium

dodecyl sulfate solution (CH3(CH2)11·OSO3Na) were

purchased from Sigma-Aldrich Co. Potassium

hydroxide (KOH) was obtained from DaeJung

Chemicals Ltd. All the chemicals were of analytical

grade and used without further purification in the

experiments.

2.2. Synthesis of burl-like amorphous NiWO4

NSs/CF

Burl-like amorphous NiWO4 NSs were synthesized

on the seed coated CT substate using a two-electrode

system based ED method. Here, we used a

commercially available and highly flexible CF as a

substrate, which is composed of non-woven copper

(Cu) plated PET (i.e., Cu/PET) fibers. Prior to the

synthesis, CF substrates (2 cm × 3 cm) were

ultrasonically cleaned in acetone, ethanol, and

de-ionized (DI) water for 5 min, respectively, and

air-dried. To prepare the seed solution, 10 mM of

Ni(CH3CO2)2·4H2O was dissolved in 100 ml of

n-propanol at room temperature and 750 µ l of

CH3(CH2)11·OSO3Na solution was slowly dropped

into the above solution using a micropipette. After 30

min with an intense magnetic stirring, the propanol

solution was changed to transparent green color.

Then, the CF pieces were soaked in the seed solution

for 15 min and placed in an oven at 120 C for 2 h to

ensure the strong adhesion of seed layer on the fibers

of CF. A simple two-electrode system with the seed

coated CF substrate as a working electrode and

platinum (Pt) mesh as a counter electrode were

utilized [53]. Meanwhile, the growth solution was

prepared by dissolving the 5 mM of Na2WO4·2H2O

and 5 mmol of Ni(NO3)2·6H2O in 900 ml of DI water

on hot plate at ~ 80-82 C. After stirring for 10 min,

the two electrodes were carefully immersed into the

aqueous growth solution with a distance of ~ 1 cm

between the electrodes. Then, the ED process was

performed at an external cathodic voltage of -1.8 V

for 15 min using a DC power supply. After

deposition, the sample was carefully removed, rinsed

with DI water, and subsequently dried at room

temperature. The weight of the as-grown sample on

CF substrate was calculated by its weight difference

before and after the ED process using an electronic

analytical balance (OHAUS DV214C), indicating ~ 0.6

± 0.03 mg/cm2.

2.3. Characterization

The surface morphology and structural properties of

the prepared samples were characterized by using a

field-emission scanning electron microscope

(FE-SEM; Carl Zeiss, LEO SUPRA 55, Reutlingen,

Germany) and transmission electron microscope

(TEM; JEM 200CX, JEOL, Tokyo, Japan) equipped

with energy dispersive X-ray spectroscopy (EDX).

The amorphous nature, chemical compositions and

surface electronic states of the prepared samples

were analyzed from the X-ray diffraction (XRD;

M18XHF-SRA, Mac Science Ltd., Yokohama, Japan)

pattern and X-ray photoelectron spectroscopy (XPS;

MultiLab2000, Thermo VG Scientific System, U.K).

The sheet resistance of the CF was measured using a

four-point probe system (FPP-RS8 Dasol Eng.).

2.4. Electrochemical Measurements

The burl-like amorphous NiWO4 NSs/CF substrate



4 Nano Res.

was directly utilized as the working electrode for the

following electrochemical measurements by cyclic

voltammetry (CV), galvanic charge-discharge (GCD)

and electrochemical impedance spectroscopy (EIS)

conducted with IviumStat electrochemical

workstation (IVIUM Technologies, Eindhoven, The

Netherlands). The electrochemical tests were

performed in a conventional three-electrode cell

beaker with a working electrode, Pt wire and

Ag/AgCl electrode as the counter and reference

electrodes, respectively, at room temperature. The

freshly prepared 1 M KOH solution served as an

electrolyte. The CV measurement was analyzed

between -0.1 V to 0.6 V at different scan rates of 5 to

70 mV/s and the GCD analysis was carried out at

different current densities of 2 to 20 A/g, respectively.

The specific capacitance (F/g) and current density

(A/g) were calculated based on the mass of electrode

material, i.e., burl-like amorphous NiWO4 NSs

weight on CF substrate. The EIS measurement was

performed in the frequency range from 100 kHz to

0.01 Hz at the open circuit potential with an AC

voltage of 5 mV.

3. Results and discussion

Figure 1 illustrates the schematic diagram of the

fabrication process of burl-like amorphous NiWO4

NSs/CF using a facile and low-cost ED method: (a)

preparation and cleaning process of CF substrate, (b)

seed layer coated CF substrate, and (c) hierarchical

and burl-like amorphous NiWO4 NSs grown on the

seed-coated CF substrate. We had chosen a

commercially available CF substrate, which was

composed of non-woven Cu plated PET fibers (i.e.,

Cu/PET fibers). The sheet resistance of the CF was

measured to be very low of ~ 0.034 Ω/sq, which is

comparable with typical metal foils and carbon

textiles as shown in Figure S2. The highly conductive

nature and flexible properties of CF can be used as a

working electrode for the growth of various metal

hydroxide/oxide nanostructures using the ED

process as shown in Figure 1(a). In addition, the large

surface area and disorderly arranged non-woven

Cu/PET 3D fibrous framework of CF facilitate the

high-speed electron transport and serve as a

cost-effective and flexible electrode for wearable

energy storage devices. After immersing the

ultrasonically cleaned CF substrate into the seed

solution, followed by thermal treatment, the nickel

seed layer was uniformly coated on the surface of CF

(Figure 1b). It has been known that, in ED process,

the seed layer plays an important role in providing

the nuclei sites, indicating that the NSs could be

grown with good adhesion on CF substrate. As

shown in Figure 1(c), when the seed coated CF was

immersed into the electrochemical setup with an

applied external cathodic voltage to the working

electrode, the amorphous NiWO4 NSs with burl-like

morphologies were grown on the surface of CF. The

mechanism for the formation of amorphous NiWO4

NSs/CF may involve the electrochemical reactions

and subsequent precipitation as described by the

following equations:

NO3- + H2O + 2e- → NO2- + 2OH-,

Ni+2 + 2OH- → Ni(OH)2,

Na2WO4 → 2Na+ + WO4-2,

Ni(OH)2 + WO4-2 + 2Na+ → NiWO4 + 2NaOH

Due to the external cathodic voltage (-1.8 V for 15

min) in growth solution, the nitrate (NO3-) ions

pertained Ni(NO3)2·6H2O was electrochemically

reduced with water on the surface of seed coated CF

substrate accompanied with the production of

hydroxyl (OH-) ions. Afterwards, the nickel (Ni+2)

was diffused onto the seed layer by columbic

attraction under strong electric field and combined

with OH- ions, resulting in the formation of nickel

hydroxide (Ni(OH)2) units on working electrode [54].

In the meantime, the generated tungstate (WO4-2)

ions from the Na2WO4 rapidly reacted with Ni(OH)2

units, leading to the successful deposition of

amorphous NiWO4 NSs/CF substrate. Figure 2 shows

the FE-SEM images and EDX spectra of the

as-prepared amorphous NiWO4 NSs on the seed

coated CF substrate under the external cathodic

voltage of -1.8 V for 15 min. As shown in the

perspective view of FE-SEM image in Figure 2(a), the

CF substrate was weaved with irregularly arranged

non-woven Cu/PET fibers and they were fully

covered by the as-prepared sample. In addition, the

texture of CF substrate was kept unchanged even

after the ED process (see the bare FE-SEM image in

Figure S1). As shown in the inset of Figure 2(b), the

porous Cu/PET fibrous framework was uniformly


5 Nano Res.

(a)Conductive fabric

(i.e., CF)

PET

fiber

Cu layer

Burl-like

NSs

Amorphous

NiWO4

nanostructures

(i.e., NSs) on CF

(b)

(c)

Seed coated CF ED process

Seed coating

& thermal treatment

Seed coated

CF fibers

Figure 1 Schematic diagram for the fabrication process of burl-like amorphous NiWO4 NSs on flexible CF substrate.

deposited over the entire surface with amorphous

NiWO4. Here, each fiber had an average diameter of

~ 20-22 m after the coating of the sample. From the

magnified view, the amorphous NiWO4 NSs were

densely and abundantly coated on Cu/PET fibers

with a burl-like morphology, as can be clearly

observed in Figure 2(b). The appearance of these NSs

on Cu/PET fibers was similar to the naturally grown

burls on trunk part of a tree. These burl-like

morphologies of amorphous NiWO4 NSs have an

average diameter of ~ 400-600 nm, as shown in the

magnified view of FE-SEM image in Figure 2(c).


6 Nano Res.

(a) (b)

(c)

W

W W W

W

NiO

Ni

Cu

Cu Cu

C

(g)

Burl-like

amorphous

NiWO4 NSs

Burl-like

amorphous

NiWO4 NSs on

CF fibers

Hierarchical

NSs

(d)

(e) (f)

Bare CF

Amorphous

NiWO4 NSs

on CF

200 nm

10 m

50 nm

20 m100 m

Figure 2 (a-c) FE-SEM images of the burl-like amorphous NiWO4 NSs/CF synthesized at an external cathodic voltage of -1.8 V for 15

min, (d) schematic illustration of the burl-like amorphous NiWO4 NSs grown on CF fibers, and (e-f) photographic images of the bare CF

and amorphous NiWO4 NSs/CF substrates. (d) EDX spectrum of the corresponding NiWO4 NSs/CF sample.

Moreover, the amorphous NiWO4 NSs are composed

of ultrathin nanosheets with thickness of several

nanometers to form a hierarchical and

three-dimensional (3D) nanonetwork (inset of Figure

2(c)). In fact, such hierarchical NSs would

significantly increase the surface area of electrode

material and enable the fast diffusion of electrolyte

through the electrodes for enhanced electrochemical

properties including high specific capacitance and

excellent cyclic stability. The assembled hierarchical

amorphous NiWO4 NSs with burl-like morphologies

on the fibers of CF are schematically shown in Figure

2(c). From the photographic images of Figures 2(f)

and 2(e), the CF was uniformly coated with a

greenish-white color product after ED process. And,

the amorphous NiWO4 NSs/CF were highly flexible

and durable without formation of any cracks on its

surface, confirming the robust adhesion of sample on

CF (Figure 2(f)). The EDX spectrum in Figure 2(g)

demonstrates the basic elemental compositions for

the amorphous NiWO4 NSs/CF. In the EDX spectrum,

the elements of Cu and C from the CF substrate were

shown, while the Ni, W, and O elements were

observed for the amorphous NiWO4 NSs. Figure 3


7 Nano Res.

1000 800 600 400 200 0

Ni 3s

Ni 3p

W 4f

(c)

W 4p

W 4d

C 1s

O 1s

Inte

nsit

y (

a.u

.)

Binding Energy (eV)

Ni 2p

10 20 30 40 50 60 70 80 90

(a)

Cu: #85-1326

(220)

(200)

Cu (111)

PET

Amorphous

NiWO4 NSs/CF

Bare CF

Inte

sit

y (

a.u

.)

2 (degree)

880 870 860 850

Ni 2p

Ni 2p1/2

"Sat""Sat"

Ni 2p3/2

Inte

nsit

y (

a.u

.)

Binding Energy (eV)

(d)

45 40 35 30

(e)

Inte

nsit

y (

a.u

.)

Binding Energy (eV)

W 4f7/2

W 4f5/2

W 4f

10 20 30 40 50 60 70 80 90

(04

1)(1

30

)(0

22

)- (10

2)

(00

2)(0

02

)(0

20

)

-(111)

(01

0)

(11

0)

(10

0)

Inte

nsit

y (

a.u

.)2 (degree)

NiWO4 NSs

JCPDS# 15-0755(b)

(01

1)

-(1

12

)

- (20

2)

- (11

3)- (3

11

)- (3

02

)

10 20 30 40 50 60 70 80 90

(041)(130)

(022)-

(102)(002)(0

02)

(020)

-(111)

(010)

(110)

(100)

Inte

nsit

y (

a.u

.)

2 (degree)

NiWO4 NSs

JCPDS# 15-0755

(b)

(011)

-(1

12)

-(2

02)

-(1

13)

-(3

11)

- (302)

540 535 530 525

295 290 285 280

Inte

ns

ity

(a

.u.)

Binding Energy (eV)

C 1s

O 1s

Inte

nsit

y (

a.u

.)

Binding Energy (eV)

(f)

Figure 3 (a-b) XRD patterns and (c-f) XPS spectra of the burl-like amorphous NiWO4 NSs/CF.

shows the XRD and XPS results of the as-prepared

sample. Figure 3(a) shows the XRD patterns of the

bare CF and the amorphous NiWO4 NSs/CF.

Obviously, the XRD patterns only revealed that CF

substrate peaks (JCPDS# 85-1326) and peaks for

NiWO4 were not observed in the XRD pattern of

NiWO4 NSs/CF which indicates the characteristic

amorphous nature of sample. As shown in the XRD

spectrum of Figure 3(b), the coated sample was

further scrapped off from the CF and calcined at 500

C for 2 h. The observed major diffraction peaks

located at 2θ degrees of 15.6, 19.27, 23.96, 24.9, 30.9,

36.5, 41.6, 52.3, 54.6, 62.3, 65.8, and 72.6 correspond

to the respective crystal planes of (010), (100), (011),

(110), (1 11), (002), (1 02), (130), (2 02), (1 13), (311), and

(302), respectively. This means that the material is

only composed of NiWO4 (JCPDS# 15-0755) without

any impurities. After calcination, the XRD spectra

and corresponding SAED pattern (Figure S3)

confirms the crystalline properties of the NiWO4 NSs.

For psuedocapacitor applications, Xing et al. and Niu

et al. recently reported that amorphous

characteristics of the metal tungsten tetraoxide

nanomaterials are beneficial for improving the

electrochemical properties, because the poor

crystalline material may exhibit faster charge transfer

reactions by providing the increased transport

channels in electrolyte ions than the crystalline one

[49, 50]. The surface bonding and element oxidation

states of the as-prepared sample were further

characterized by XPS spectra and the results were

plotted in Figures 3(c-f). As observed in Figure 3(c),

the XPS survey scan spectrum indicates the presence

of Ni, W, O and C elements with their respective

binding energies. Herein, C 1s peak appears mainly

due to the fortuitous carbon formed during

atmospheric air exposure. The high-resolution Ni 2p,

W 4f, and O 1s spectral curves were well-fitted with

Gaussian curves as shown in Figures 3(d-f). From the

Figure 3(c), the Ni 2p spectrum showed two-spin

orbit doublets and shake-up satellite peaks observed

in the range of 850-884 eV. The Ni 2p spectrum had

binding energies of 855.1 and 871.6 eV which are the

characteristic peaks of Ni 2p3/2 and Ni 2p1/2 spin-orbit

doublets, respectively. These results reveal that the

Ni species are in +2 oxidation state. Meanwhile, the

high-resolution XPS spectrum of W 4f appears as the

spin-orbit splitting of W 4f7/2 at 36.4 eV and W 4f5/2 at


8 Nano Res.

34.6 eV, implying that the W is in +6 oxidation state

in the prepared product. Also, the O 1s spectrum

with a binding energy value of 530.8 eV was

associated to the bound O-bond with W and Ni

species in NiWO4 [23, 48]. Furthermore, the atomic

ratio of Ni to W was close to 1:1 with a stoichiometry

ratio of the amorphous NiWO4 NSs/CF. Combining

the XPS result with that XRD result confirms the

successful formation of amorphous NiWO4 NSs/CF

without any impurities.

The hierarchical structures of amorphous

NiWO4 NSs were further investigated by the TEM

images. To prepare the sample for TEM analysis,

amorphous NiWO4 NSs/CF substrate was cut into

small pieces and they were agitated in a small beaker

containing 10 ml of ethanol for 30 min by

ultrasonication. Then, the Cu grid mesh was

immersed into the ethanol solution and air-dried for

5 min before putting it into the TEM chamber. The

typical TEM images of hierarchical amorphous

NiWO4 NSs separated from the CF substrate were

shown in Figures 4(a-d). In TEM image of the Figure

4(a), amorphous NiWO4 NSs showed flower shaped

structures after detached from the CF and they were

agglomerated with each other. The average diameters

of these NSs were approximately 400-500 nm. The

TEM image of a single amorphous NiWO4 NS in

Figure 4(b) further reveals that the NSs were

well-assembled with 3D ultrathin nanosheets. Such

hierarchical features of NSs not only provide the

channels for diffusion of electrolyte ions into the

interior parts of the material, but also increase the

surface area for rapid faradic redox reactions, when

they were used as a pseudocapacitive material. The

high-resolution TEM image, as shown in Figure 4(c),

clearly shows no obvious lattice fringes on the

surface of NiWO4 NSs. Moreover, the corresponding

selected area electron diffraction (SAED) pattern was

taken from the area in Figure 4(c), confirming that

there are no crystalline ring patterns or dotted spots,

undeniably proving an amorphous phase of NiWO4

NSs, which agrees well with the XRD pattern of

Figure 3(a). The TEM-EDX mapping was performed

to illustrate the spatial distribution of the

corresponding elements in the burl-like amorphous

NiWO4 NSs. As shown in Figures 4(e-g), the EDX

mapping images clearly revealed the uniform

distribution of Ni (green), W (blue) and O (red)

atoms within the whole NiWO4 NSs.

The electrochemical properties of the burl-like

amorphous NiWO4 NSs/CF were investigated using

(a) (b) (c)

W NiO

(d)(e)(f)(g)

200 nm 100 nm 5 nm

5 1/nm

10 nm

Figure 4 (a-d) TEM images and (e-f) EDX elemental mapping images of the hierarchical amorphous NiWO4 NSs


9 Nano Res.

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6-60

-40

-20

0

20

40

60

Bare CF

Amorphous

NiWO4 NSs/CF

Scan rate: 30 mV.s-1

Cu

rren

t (m

A)

Potential (V, vs. Ag/AgCl)

(a)

0 5 10 15 20 25200

400

600

800

1000

1200

1400

Sp

ecif

ic C

ap

acit

an

ce (

F/g

)

Current Density (A/g)

(d)

0 100 200 300 400 500 600 7000.0

0.1

0.2

0.3

0.4

0.5

Po

ten

tia

l (V

, vs.

Ag

/Ag

Cl)

Time (sec)

2 A/g

4 A/g

6 A/g

8 A/g

10 A/g

15 A/g

20 A/g

(c)

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

-100

-75

-50

-25

0

25

50

75

100

0 10 20 30 40 50 60 70-100

-75

-50

-25

0

25

50

75

100

Pe

ak

Cu

rre

nt

(mA

)

Scan Rate (mV/s)

Anodic current

Cathodic current

50 mV/s

70 mV/s

Cu

rren

t (m

A)

Potential (V, vs. Ag/AgCl)

5 mV/s

10 mV/s

20 mV/s

30 mV/s

40 mV/s

(b)

Figure 5 (a) CV curves of the bare CF and the amorphous NiWO4 NSs/CF measured at a scan rate of 30 mV/s. (b) CV curves of the

amorphous NiWO4 NSs/CF at different scan rates (5 to 70 mV/s). Inset of (b) shows the plot of redox peak currents with respect to scan

rate. (c) GCD curves of the amorphous NiWO4 NSs/CF at different current densities (2 to 20 A/g). (d) Calculated specific capacitance

values as a function of applied current density.

CV, GCD and EIS measurements in three-electrode

cell system with 1 M KOH aqueous solution. Figure

5(a) shows the CV curves measured for the bare CF

and the amorphous NiWO4 NSs/CF electrode at a

scan rate of 30 mV/s with a potential window of -0.1

to 0.6 V vs. Ag/AgCl electrode. Apparently, the area

under CV curve for the bare CF was very small,

which indicates that the contribution of bare CF to

the capacitance was negligible. Meanwhile, under the

same scan rate for the amorphous NiWO4 NSs/CF

electrode, relatively larger CV integrated area was

observed, exhibiting the superior capacitive behavior

of the materials. Moreover, the CV curve of the

amorphous NiWO4 NSs/CF electrode indicates the

presence of redox peaks, which is probably due to

the charge storage kinetics of NiWO4 NSs originating

from the pseudocapacitive behavior (Faradic redox

reactions). The corresponding redox process at the

NiWO4/electrolyte interface can be expressed as

follows [55]:

Ni+2 ↔ Ni+3 + e-.

Herein, the pseudocapacitance is mainly based on

the Faradic redox reactions of Ni ions in the NiWO4

nanomaterial. The W species in the product have

only contributed to the enhancement of electrical

conductivity, but not been involved in any redox

reactions [50]. Figure 5(b) shows the CV curves of

amorphous NiWO4 NSs/CF at different scan rates of

5, 10, 20, 30, 40, 50, and 70 mV/s in the potential

window of -0.1 to 0.6 V. As can be seen in the shape

of CV curves, the redox peaks were observed to be

mirror-image symmetric against each other for all the


10 Nano Res.

scan rates, indicating the good electrochemical

reversibility of the as-prepared product [56]. As the

scan rate increased, the redox peak positions were

shifted slightly to the higher and lower potentials

with enhanced anodic and cathodic peak current

values. The linear relationship between increased

scan rates and current values of the redox peaks was

shown in the inset of Figure 5(b). This indicates that

the electrochemical process is a diffusion-controlled

process [57]. To estimate the superior electrochemical

performance in detail, the GCD measurements were

carried out in 1 M KOH electrolyte solution at room

temperature. Figure 5(c) shows the GCD curves of

the amorphous NiWO4 NSs/CF electrode at various

current densities. During the charging and

discharging phenomenon in all GCD curves, the

charge curves were almost similar to their respective

discharge counterpart, which was in well agreement

with the CV results. With regard to the GCD curves,

the specific capacitance of the amorphous NiWO4

NSs can be calculated using the following formula

[36, 39]:

C = (i×∆t)(m×∆V),

where C is the specific capacitance (F/g), i is the

discharge current (A), ∆t is the discharge time (sec),

m is the mass of amorphous NiWO4 NSs/CF

electrode (g), and ∆V is the potential window of

discharge curve (V). The calculated specific

capacitance values were 1190.2, 1065.1, 985.3, 921.6,

882.2, 790.5, and 765.7 F/g at the current densities of

2, 4, 6, 8, 10, 15, and 20 A/g, respectively. These

enhanced specific capacitance values were probably

attributed to the reliable adhesion of burl-like

amorphous NiWO4 NSs/CF substrate, which results

in an intimate contact and rapid electron transport

between the CF and each NS. Also, the porous

fibrous framework of CF substrate and high specific

surface area of the amorphous NiWO4 NSs possibly

initiated the hierarchical pathways for efficient

electrolyte ion transport. Moreover, the specific

capacitance values observed in the present work are

comparable to or even better than those of

previously reported metal tungsten tetraoxide

based electroactive materials such as

cauliflower-like amorphous NiWO4 NSs (586.2 F/g

at 0.5 A/g and 376.9 F/g at 8 A/g) [52], DNA

encapsulated chain-like NiWO4 NSs (173 F/g at 5

mV/s) [50], amorphous CoWO4 nanoparticles (403

F/g at 0.5 A/g and 298 F/g at 3 A/g) [51], and

rGO-CoWO4 nanocomposite (159.9 F/g at 5 mV/s)

[58], respectively. The specific capacitance values as

a function of applied current density (2 A/g to 20

A/g) were plotted in Figure 5(d). The burl-like

amorphous NiWO4 NSs/CF electrode exhibited

excellent capacity retention rates of 74.1 and 64.3%

at the current densities of 10 and 20 A/g as

compared with the initial current density of 2 A/g.

This decreased capacitance under high current

densities can be explained by the fact that the

electrolyte ions are only diffused to the outer

surface of the electroactive material at high applied

current densities. Meanwhile, at low current

densities, the electrolyte ions were easily diffused to

both inner and outer surface of the electroactive

material to be effectively involved in the reversible

electrochemical reactions.

The fundamental electrochemical behavior of

the as-prepared burl-like amorphous NiWO4 NSs/CF

was measured by the EIS analysis. The measurement

was performed in an open circuit potential at the

frequency range of 100 kHz to 0.01 Hz with an AC

voltage of 5 mV. As shown in Figure 6(a), the EIS

curve is a plot of the imaginary part (Z”) of the

impedance against the real part (Z’), which can be

characterized by two distinct parts: a negligible

semicircle in the high-frequency region followed by a

sloped line in the low-frequency region. The

diameter of the semicircle in the low-frequency

region corresponds to the Faradic reactions at the

electrode/electrolyte interface, whereas the sloped

line is associated with the faster diffusion of

electrolyte ions [59]. The inset of Figure 6(a) shows

the equivalent circuit used to fit the EIS curves,

where Rs, Rct, CPE, and Zw correspond to internal

resistance, interfacial charge transfer resistance, the

constant phase element, and Warburg impedance,

respectively. The low values of Rs and Rct were found

to be 1.85 and 2.36 Ω for the amorphous NiWO4

NSs/CF, indicating the good conductivity of

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

OH-

OH-

OH-

OH-

OH-

OH-

OH-

OH-

OH-

e-

e-e-

OH-

OH- OH-

OH-

Electrolyte

penetration

Electron

transfer

10 m

0 20 40 60 800

20

40

60

80

0 2 4 6

0

2

4

6

8

Z''(

oh

m)

Z'(ohm)

W

RS

CPE

CL

RctZW

Z''(o

hm

)

Z'(ohm)

(a)

0 100 200 300 400 500 600 700 8000.0

0.1

0.2

0.3

0.4

0.5

Po

ten

tia

l (V

, v

s.

Ag

/Ag

Cl)

Time (sec)

Current Density: 10 A/g(c) (d)

0 200 400 600 800 10000

200

400

600

800

1000

Sp

ec

ific

Ca

cit

an

ce

(F

/g)

Cycle Number

Current density: 10 A/g(b)

OH-

e-

Figure 6 EIS spectrum of the amorphous NiWO4 NSs/CF in 1 M KOH electrolyte solution. (b) Cycling performance of the burl-like

amorphous NiWO4 NSs/CF at a current density of 10 A/g and (c) corresponding GCD curves for the first 10 cycles. (d) Schematic

illustration of the electrolyte diffusion and electron transport path way of the burl-like amorphous NiWO4 NSs/CF. The inset of (b)

shows the FE-SEM image of the amorphous NiWO4 NSs/CF after 1000 cycles, revealing its structural stability

as-fabricated material. The long-term cycling

performance is also one of the key parameters in

determining the psuedocapacitors for practical

applications. Herein, the cycling stability was carried

by repeated GCD measurements at a constant current

density of 10 A/g for 1000 cycles in 1 M KOH

electrolyte solution, as shown in Figure 6(b). It was

found that the capacitance retention was observed to

be 92 % after 1000 cycles, which indicates the good

cyclic stability of the burl-like amorphous NiWO4

NSs/CF. Figure 6(c) shows the repeated GCDcurves

(first 10 cycles) for the amorphous NiWO4 NSs/CF at

a current density of 10 A/g. Visibly, all the GCD

curves remain undistorted and display a symmetrical

potential-time response behavior, exhibiting the

highly reversible characteristics of the active material.

After 1000 cycles, the burl-like amorphous NiWO4

NSs were kept intact on CF fibers (inset of Figure 6(b))

and the surface morphology of hierarchical

amorphous NiWO4 NSs appeared similar to its initial

morphology as shown in Figure S4 (a). Also, during

the cycling process in electrolyte solution, the

amount of NiWO4 NSs on CF still remains without

any mass loss or peeling as shown in Figure S4 (b),

which confirms the excellent stability and reliable

adhesion of burl-like amorphous NiWO4 NSs to the

CF. To further confirm the structural properties after

cycling process, the XPS analysis was carried out for

the burl-like amorphous NiWO4 NSs/CF. From the

XPS survey scan spectrum of Figure S4 (c), the

expected elements of Ni, W and O were well indexed

to their respective binding energy values, which


12 Nano Res.

clearly reveals the reliable structural stability of the

burl-like amorphous NiWO4 NSs/CF even after

long-term cycling process. As schematically shown in

Figure 6(d), the burl-like amorphous NiWO4 NSs

entrapped on CF substrate using a facile ED process

possess several advantageous features and properties

to enhance the electrochemical properties as a

psuedocapacitor electrode: (i) an express pathway for

efficient electron transport particularly owing to an

intertwined fibrous framework and largely extended

surface area of flexible CF substrate; (ii) good

electrical conductivity, high specific surface area, and

hierarchically nanostructured burl-like amorphous

NiWO4 on CF as backbone for electron transport and

electrolyte diffusion by providing a greater number

of electroactive sites and ionic transportation

channels for reversible electrochemical reactions; (iii)

direct growth and robust adhesion of burl-like

amorphous NiWO4 NSs/CF substrate for the polymer

binder- and conductive additive-free integrated

electrode for psuedocapacitors, which reduces

additional interfacial resistances and defects, thus

initiating the electroactive material for the charge

storage. This facile fabrication of hierarchical

amorphous NiWO4 NSs with burl-like morphologies

on CF could be used as a promising electrode for

flexible energy storage devices.

The electrochemical performance of the

electrochemical cell was also investigated by

calculating energy density and power density of the

burl-like amorphous NiWO4 NSs/CF electrode.

Generally, a good electrochemical psuedocapacitor is

expected to provide high specific capacitance and

high energy density at rapid charging-discharging

rates. Using GCD curves at different current densities

in Figure 5, the energy density and power density of

the burl-like amorphous NiWO4 NSs/CF were

calculated using the following formulas [30]:

E = C∆V2,

P = E/∆t.

Here, E is the energy density (Wh/kg), C is the

specific capacitance (F/g), ∆V is the potential window

(V), P is the power density (W/kg), and Δt is the

discharge time (s). The obtained energy density and

power density values were plotted in Ragone chart as

shown in Figure 7(a). The maximum energy density

was 33.47 Wh/kg at a power density of 125.18 W/kg

and significantly, it still remains 18.93 Wh/kg at a

power density of 1173.84 W/kg, confirming that the

burl-like amorphous NiWO4 NSs/CF can be

promising electrode material for high-performance

psuedocapacitor applications. In order to show the

flexible property, the working electrode (0.5 × 3 cm2)

was cut and bended with a help of cotton thread as

depicted in the schematic and photographic images

of Figure 7(b). In the photographic images of Figure

7(c), the electrochemical properties of the burl-like

amorphous NiWO4 NSs/CF electrode under normal

and bending positions were carried out using a three

electrode system in 1 M KOH electrolyte solution.

Figure 7(d) shows the CV curves for the burl-like

amorphous NiWO4 NSs/CF under normal and

bending positions at a scan rate of 30 mV/s.

Apparently, the integral area under the CV curves

does not change and the redox peak positions were

similar in the normal and bending positions of the

pseudocapacitive electrode. This can be attributed to

the reliable adhesion and mechanical stability of the

burl-like amorphous NiWO4 NSs/CF electrode.

Therefore, the electrolyte ions can be easily diffused

into the active material for reversible electrochemical

reactions even under flexible condition. To further

study the electrochemical performance of the

burl-like amorphous NiWO4 NSs/CF electrode under

normal and bending conditions, GCD measurements

were conducted at current density of 5 A/g. As seen

in Figure 7(e), the charge-discharge times were

almost overlapping with each other, which clearly

indicates their charge storage performance was

constant in normal and bending conditions,

respectively. Moreover, the Rct values were kept

unchanged during normal and bending conditions

from the EIS curves (Figure 7(f)), which further

confirm the good electrochemical reliability of

flexible electrode. The higher mechanical flexibility

and good electrochemical performance of the burl-

like amorphous NiWO4 NSs/CF electrode ensures its

potential use for flexible and wearable energy storage

device applications.

Needle

Bended sample

Cotton thread

100 10001

10

100

En

erg

y D

en

sit

y (

Wh

/kg

)

Power Density (W/kg)

(a)

Cotton

thread

CF

(electrode)

NiWO4 NSs

0.5 cm2

0.5 2 cm2

(b)

0 40 80 120 160 2000.0

0.1

0.2

0.3

0.4

0.5

Bending

Normal

Po

ten

tia

l (V

, v

s.

Ag

/Ag

Cl)

Time (s)

(e)

0 20 40 60 80 1000

20

40

60

80

100

Z'(ohm)

Normal

Bending

Z''(

oh

m)

(f)

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6-60

-40

-20

0

20

40

60

Normal

Bending

Cu

rren

t (m

A)

Potential (V vs. Ag/AgCl)

(d)

Bending

Working

electrodeAg/AgCl

electrode

Pt wire

Normal

Working

electrodeAg/AgCl

electrode

(c)

Figure 7 (a) Ragone plot of the burl-like amorphous NiWO4 NSs/CF electrode obtained from GCD curves, (b) schematic and

photographic images of the preparation of working electrode for bending test and (c) photographic images of the burl-like amorphous

NiWO4 NSs/CF electrode under normal and bending conditions in three electrode cell system with 1 M KOH electrolyte solution. (d-f)

Electrochemical performance of the burl-like amorphous NiWO4 NSs/CF electrode under normal and bending conditions.

4. Conclusion

In summary, hierarchical and burl-like

amorphous NiWO4 NSs were facilely fabricated on

flexible CF substrate via a simple two-electrode

system based ED method. Herein, under the external

cathodic voltage (-1.8 V for 15 min), direct growth of

amorphous NiWO4 NSs/CF with burl-like

morphologies was carried out and it was used as the

binder- and conductive additive-free electrode for

psuedocapacitors. The designed flexible electrode in

this work exhibited the highest specific capacitance

of 1192.2 F/g at the current density of 2 A/g and the

capacitance loss was observed to be very low (8 %) at

a current density of 10 A/g after 1000 cycles, which

are higher among the NiWO4 NSs based electroactive

materials for psuedocapacitors. By the facile growth,

the amorphous NiWO4 NSs/CF based

psuedocapacitor electrode can be further expanded

to other metal tungsten tetroxide based materials for

enhanced energy storage and photocatalytic

applications.

Acknowledgements

This work was supported by the National Research

Foundation of Korea (NRF) grant funded by the

Korea government (MSIP) (No. 2014-069441).

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Electronic Supplementary Material: Supplementary material (FE-SEM images of the bare CF which shows the

conductive metallic layer of Cu on the PET fibers surface) is available in the online version of this article at

http://dx.doi.org/10.1007/s12274-***-****-* (automatically inserted by the publisher).

http://dx.doi.org/10.1007/s12274-***-****-*


Nano Res.

Electronic Supplementary Material




Goli Nagaraju1, Ramesh Kakarla2, Sung Min Cha1 and Jae Su Yu1 ()

Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)

Figure S1 FE-SEM images of the bare CF with different magnifications, where it was plated with metallic layer of Cu on PET fibers.

20 m 5 m

50 m

5 m

(a)

(b)

(c)

PET

fiber

Cu

layer

Bare conductive fabric (CF)


Nano Res.

Figure S2. Sheet resistance of the various conductive electrodes measured using four-point probe system.


Copper

foil

Nickel

foil

Conductive

fabric (CF)Graphite

sheet

Carbon

cloth

Copper foil Nickel foilConductive

fabric (CF)

Carbon

clothSubstrate

Graphite

sheet

28-28.5 28-28.7 34-35.5 41-42.1 1140-1270

Sheet

resistance

(mΩ/sq)


Nano Res.

Figure S3. SAED pattern of the NiWO4 NSs heated at 500 C for 2 h, showing its crystalline properties.

5 1/nm


Nano Res.

Figure S4. (a) FE-SEM images of the burl-like amorphous NiWO4 NSs/CF after 1000 cycles, (b) photographic images of the working

electrode during cycling process, showing the strong adhesion of burl-like amorphous NiWO4 NSs to the CF without any peeling and (c)

XPS analysis of the burl-like amorphous NiWO4 NSs/CF after long-term cycling process.

1000 800 600 400 200 0

Ni 3p

Ni 3s

C 1s

W 4dW 4f

W 4p

O 1s

Ni 2p

Binding Energy (eV)

Inte

nsit

y (

a.u

.)

(c)

50 nm

200 nm

(a) (b)

Working

electrode

Ag/AgCl

electrode

Pt wire

highly flexible conductive fabrics with hierarchically ... · highly flexible conductive fabrics...

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